This invention relates to the methods and apparatus for the separation of isotopes or other atomic or molecular species which are difficult to separate. Such isotope separation has wide application in the fields of radiation chemistry, in the production of radioactive isotopes, and in the production of nuclear reactor fuels. In the production of nuclear fuels, diffusion and centrifuge processes have long been used, but have been costly to implement. In the production of radioisotopes, particularly those of short lifetime for medical purposes, past processes are often too long compared to useful lifetimes of certain isotopes.
Proposals have been made for other techniques of isotope separation as by the use of lasers. Reference is made to the review article by C. Bradley Moore, "The Application of Lasers to Isotope Separation," in the Accounts of Chemical Research, Volume 6, page 223 et seq., published January, 1973, for a review of such techniques. In particular, one technique which has received considerable attention is that described by R. V. Ambartzumian and V. S. Letokhov in Applied Optics, Volume 11, page 354, published February, 1972, "Selective Two-Step (STS) Photoionization of Atoms and Photodissociation of Molecules by Laser Radiation," hereinafter referred to as STS. These previously proposed techniques will now be reviewed as applied to the separation of U235 and U238. The STS proposal provides that the U235-U238 mixture would be vaporized and then excited with a tunable laser tuned to the resonant frequency of the ground to excited state transition of the desired isotope species, after which the excited species would be photoionized and separated from the other species. Due to what is called isotopic shift, the resonant frequency of one isotope typically differs from an adjacent isotope by a small fraction of the wave number to many wave numbers. For example, for uranium the U235-U238 isotope shift is about 1.4 wave numbers for the 4244.4 A transition. (H. G. Kuhn, Atomic Spectra, 2nd Ed., 1962, Academic Press, Plate 18, opposite page 335.) The U235 can therefore be selectively excited, as by a tuned, narrow band laser, to an excited state, while the other specie, U238, remains in the ground state. The laser power densities which are required for the first step of excitation are quite modest and would typically be in the range of tens of watts/cm.sup.2. Of course, the tunable laser required for the STS process must have an output linewidth that is small compared to the isotopic shift. The pressure (i.e., concentration) of the isotope mixture has to be such that exchange collisions do not dominate before the desired isotope is separated out, see the Moore review article referenced above.
In the second step of the previously proposed STS process, the selected and previously excited specie A is photoionized at a second frequency with photons having energy sufficient to photoionize the specie A, but insufficient to photoionize specie B, still in the ground state.
A general and important problem with the STS technique is that the absorption cross section seen by the second photon is typically at least five orders of magnitude smaller than the cross section available to put the desired isotopic specie into the first excited state. Because the absorptive cross section is so much smaller, it is readily shown that, if each previously excited atom is to be ionized, the incident power flux for the second photon must be increased over that for the first photon in the same ratio that this second cross section is smaller than the first cross section. For example, if the photoionization cross section is five orders of magnitude smaller than is the cross section for excitation to the first state, then five orders of magnitude more power is required in the second photon to photodissociate all of the excited specie. The fact that this greatly increased power density is required leads to an inherently inefficient process. Though the power density need be very high in order to excite all the atoms which are present, there may not be a sufficient number of atoms present to absorb a sufficient number of this incident power to allow a process with reasonable efficiency. The problem is made worse since the isotope pressure, for instance of U235-U238, must be maintained sufficiently low that exchange collisions between an excited and ground state isotopic specie do not occur faster than the desired isotope may be separated out. There is therefore a need for a new and improved method and apparatus for separating isotopes using the principle of selective isotope excitation.